Solar energy using photovoltaic effect requires active semiconducting materials to convert light into electricity. Currently, solar cells based on silicon are the dominating technology due to their high conversion efficiency. Recently, solar cells based on organic materials showed interesting features, especially on the potential of low cost in materials and processing. Judging from the recent success in organic light emitting diodes based on a reverse effect of photovoltaic effect, organic solar cells are very promising.
Organic photovoltaic cells have many potential advantages when compared to traditional silicon-based devices. Organic photovoltaic cells are light weight, economical in the materials used, and can be deposited on low cost substrates, such as flexible plastic foils. However, organic photovoltaic devices typically have relatively low power conversion efficiency (the ratio of incident photons to energy generated). This is, in part, thought to be due to the morphology of the active layer. The charge carriers generated must migrate to their respective electrodes before recombination or quenching occurs. The diffusion length of an exciton is typically much less than the optical absorption length, requiring a tradeoff between using a thick, and therefore resistive, cell with multiple or highly folded interfaces, or a thin cell with a low optical absorption efficiency.
Conjugated polymers are polymers containing π-electron conjugated units along the main chain. They can be used as active layer materials for some types of photo-electric devices, such as polymer light emitting devices, polymer solar cells, polymer field effect transistors, etc. As polymer solar cell materials, conjugated polymers should possess some properties, such as high charge carrier mobility, good harvest of sunlight, good processability, and proper molecular energy levels. Some conjugated polymers have proven to be good solar cell materials. Conjugated polymers are made of alternating single and double covalent bonds. The conjugated polymers have a δ-bond backbone of intersecting sp2 hybrid orbitals. The pz orbitals on the carbon atoms overlap with neighboring pz orbitals to provide π-bonds. The electrons that comprise the π-bonds are delocalized over the whole molecule. The semiconducting properties of the photovoltaic polymers are derived from their delocalized π bonds. The substituents of the polymers also largely influence the electronic properties. The optical bandgap, mobility and thin-film morphology are affected by both the type of functional group used as a substituent and the bulkiness and length of the side chain. Polymers which have only minor differences in the side chains will have large differences in the device performance.
Additionally, one way of improving device efficiency of organic photovoltaics is through utilizing interfacial charge transport layers. Interfacial charge transport layers sandwich the photoactive layer and determine the device polarity, help to collect charges, and transport the charges to the electrodes. Materials for these charge transport layers can be transparent, have low resistance and be chemically stable. The electron transport layer collects and transports electrons mainly generated from the acceptor to the cathode. A low work function interface is required to make Ohmic contact with the organic photoactive layer.
There is a need in the art for polymer solar cells that exhibit increased power conversion efficiency and fill factor and a new low temperature sol-gel solution processing technique for preparing oxides with tunable composition with cross-linkable fullerene derivatives